Why no aluminum???

[russv]

Originally Posted by thermobox

If you are concerned with weight savings in your exhaust system, you will want to wait for titanium systems to become available.

My GD Magnaflow Ti catback could be held up by one pinky finger.

I would think titanium would be God-awful-expensive

[MKIV]

Originally Posted by epson1

aluminum is very easy to weld and I've used it for exhaust before. someone should probably tell STM or Full-race they're wrong for using aluminum exhausts on their 9 sec+ race cars then....

It is the same reason as alum. con rods. Race cars okay, street no unless you like to change your rods with every oil change.

[serialk11r]

Originally Posted by MKIV

It is the same reason as alum. con rods. Race cars okay, street no unless you like to change your rods with every oil change.

I don't think that's a relevant analogy. The exhaust pipe is under very little stress. I don't doubt that an aluminum exhaust would probably start developing stress fractures or something after some time (possibly sooner than most owners would be happy with), certainly before any stainless or titanium exhaust, but since there's little structural integrity required from an exhaust pipe I think it's not fair to compare it to a critical engine component.

I have no idea what exhaust temperatures are typically, but I imagine that after the catalyst there should be enough cooling for the exhaust pipe to let it withstand the heat.

[thermobox]

Originally Posted by russv

I would think titanium would be God-awful-expensive

Yep. It was no joke being the first one on the block with the Magnaflow Ti System. 1,325 Shipped. Ouchie! Shortly there after they slashed the pricing to 7xx and then the system was discontinued due to lack of interest.
But the JDM will come correct with some different Ti systems for the BRZ. It's just a matter of time.

[xwd]

Originally Posted by thermobox

Yep. It was no joke being the first one on the block with the Magnaflow Ti System. 1,325 Shipped. Ouchie! Shortly there after they slashed the pricing to 7xx and then the system was discontinued due to lack of interest.
But the JDM will come correct with some different Ti systems for the BRZ. It's just a matter of time.

There are a couple out there now, one from Amuse and another from SARD. The Amuse is like $3200 for the axleback, I think it's close to $4k for the entire catback.

[Dimman]

Originally Posted by serialk11r

I don't think that's a relevant analogy. The exhaust pipe is under very little stress. I don't doubt that an aluminum exhaust would probably start developing stress fractures or something after some time (possibly sooner than most owners would be happy with), certainly before any stainless or titanium exhaust, but since there's little structural integrity required from an exhaust pipe I think it's not fair to compare it to a critical engine component.

I have no idea what exhaust temperatures are typically, but I imagine that after the catalyst there should be enough cooling for the exhaust pipe to let it withstand the heat.

Stress reversals from vibration will play a significant part in fatigue. Add to the fact that aluminum's strength drops at those higher temps.

I think that axle-back could be ok for a couple years, but life will get shorter the closer to the motor. But there could also be more stress and fatigue issues with a hybrid stainless/aluminum systems from different expansion rates and galvanic corrosion.

Meh...

I think from my avatar and sig people know what my choice is...

[OrbitalEllipses]

I was waiting for you Dimman!

Someone said stainless steel is the best weight/wear compromise. Not true; titanium is. Unfortunately, stainless still is the best weight/wear/price compromise while titanium still remains expensive (not to mention requiring a far more experienced and knowledgeable welder).

[Dave-ROR]

Originally Posted by serialk11r

I don't think that's a relevant analogy. The exhaust pipe is under very little stress. I don't doubt that an aluminum exhaust would probably start developing stress fractures or something after some time (possibly sooner than most owners would be happy with), certainly before any stainless or titanium exhaust, but since there's little structural integrity required from an exhaust pipe I think it's not fair to compare it to a critical engine component.

I have no idea what exhaust temperatures are typically, but I imagine that after the catalyst there should be enough cooling for the exhaust pipe to let it withstand the heat.

You are forgetting about vibration :P

An aluminum exhaust simply won't last any reasonable length of time on a street car.

Perhaps a mix of SS and AL with SS closer to the cat, but that probably has it's own issues.

Let's face it, there's plenty of reasons why manufacturers don't do it, it's not because no one has thought of it before

If you want light, Ti is the answer. It's not cheap though obviously.

[Dave-ROR]

Originally Posted by OrbitalEllipses

I was waiting for you Dimman!

Someone said stainless steel is the best weight/wear compromise. Not true; titanium is. Unfortunately, stainless still is the best weight/wear/price compromise while titanium still remains expensive (not to mention requiring a far more experienced and knowledgeable welder).

I should have just finished reading the thread before replying eh? :P

Anyways, yes, SS is the best for the money. Ti is the best otherwise for a long lasting solution.

[serialk11r]

Originally Posted by Dave-ROR

You are forgetting about vibration :P

An aluminum exhaust simply won't last any reasonable length of time on a street car.

Perhaps a mix of SS and AL with SS closer to the cat, but that probably has it's own issues.

Let's face it, there's plenty of reasons why manufacturers don't do it, it's not because no one has thought of it before

If you want light, Ti is the answer. It's not cheap though obviously.

Aluminum is more expensive than mild steel, and is less durable at temperature, that's why they don't use it in street cars.

But what's a reasonable length of time? A street car is run mostly at part load where exhaust temperatures are low. Vibration and other stresses can be dealt with in the mounting system, at least enough to extend the life of the exhaust. Where I'd be rather concerned is a racetrack, where the exhaust temperatures are consistently high enough to melt aluminum. In the future as we see forms of exhaust heat regeneration that reject exhaust at temperatures that look more like 200-300C instead of 800C, then I think aluminum would be competitive against Ti/steel in all respects.

[Dave-ROR]

Originally Posted by serialk11r

Aluminum is more expensive than mild steel, and is less durable at temperature, that's why they don't use it in street cars.

But what's a reasonable length of time? A street car is run mostly at part load where exhaust temperatures are low. Vibration and other stresses can be dealt with in the mounting system, at least enough to extend the life of the exhaust.

Between expansion/contraction and vibration I bet you'd be lucky to see 2 years of normal use. Also the risk of it running hot and developing a leak from melting. I've seen more than a few exhaust related fires (two this year racing) from exhausts cracking (accident, poor welds and simple fatique) and directing hot exhaust at fuel tanks.. boils for awhile first though :P

I would never run one. SS is fine, heavier, but low to the ground so shrug.

Cost isn't the reason, otherwise Ti exhausts would never be created The fact that it gets brittle and very soft when it gets hot is the reason. That and liability of course.

[serialk11r]

Originally Posted by Dave-ROR

Cost isn't the reason, otherwise Ti exhausts would never be created The fact that it gets brittle and very soft when it gets hot is the reason. That and liability of course.

Ti is expensive but durable as you say. Aluminum as you say, can be a disaster waiting to happen, which would never be acceptable for a commercial product. Liability at higher cost isn't worth it.

Crazy people like me though, get the idea of swapping out sections of tube for aluminum Thanks for the warning about the fuel tanks.

[OrbitalEllipses]

Originally Posted by Dave-ROR

I should have just finished reading the thread before replying eh? :P

Gettin' slow old man!

[arghx7]

Whoa, whoa... you guys realize there are millions of cars with aluminum exhaust components including turbo engines?

Originally Posted by Dimman

I think that axle-back could be ok for a couple years, but life will get shorter the closer to the motor. But there could also be more stress and fatigue issues with a hybrid stainless/aluminum systems from different expansion rates and galvanic corrosion.

This is a theory that makes sense at first glance, but when you look at real vehicles on the road today it turns out that you've got it in reverse. There are basically zero axle-back exhausts on street cars, but millions of vehicles with aluminum exhaust manifolds. Most of them are cast into the head.

I have four examples for you, two non-turbo and two turbo.

First is the Chrysler Pentastar V6 engine you find in all sorts of longitudinal configurations, like on the V6 Challenger.



next is the Honda R18 engine used in a gazillion Civics



third is the new EA888 1.8T VW engine for European markets that I have discussed in the past. This engine also features port + direct injection like the FA20:



note that this manifold is aluminum and water-cooled. It's also cast into the head. The last example is the Honda K23A1 engine found in the outgoing RDX models. It's aluminum and water cooled, but it is not cast into the cylinder head:

Originally Posted by serialk11r

Aluminum is more expensive than mild steel, and is less durable at temperature, that's why they don't use it in street cars.

But what's a reasonable length of time? A street car is run mostly at part load where exhaust temperatures are low.

Again, this seems to make sense on the surface except for the fact that exhaust temperatures have gotten very high over recent years. Engines run much leaner during transients to keep CO emissions down, as measured on the US06 test cycle. And you don't see gazillions of aluminum exhaust manifolds cracking.

The most common manifold design for modern emissions standards is probably the dual-wall stainless steel type, but aluminum exhaust manifolds are becoming increasingly more common. As for the rest of the vehicle, well I don't design catalysts but I'm sure cost is a big issue and I imagine there are a lot of problems making aluminum shells for catalysts.

Just because small companies with low production volumes haven't made it work doesn't mean there are insurmountable technical challenges.